1. Field of Invention
This invention relates to semiconductor device packages, and more particularly to grid array packages using the controlled collapse chip connection (C4) method to attach an integrated circuit (i.e., a chip) to a substrate.
2. Description of Related Art
During manufacture of an integrated circuit (e.g., a microprocessor), signal lines formed upon the silicon substrate which are to be connected to external devices are terminated at flat metal contact regions called input/output (I/O) pads. Following manufacture, the integrated circuit is typically secured within a protective semiconductor device package. Each I/O pad of the chip is then connected to one or more terminals of the device package. The terminals of a device packages are typically arranged about the periphery of the package. Fine metal wires are typically used to connect the I/O pads of the chip to the terminals of the device package. Some types of device packages have terminals called "pins" for insertion into holes in a printed circuit board (PCB). Other types of device packages have terminals called "leads" for attachment to flat metal contact regions on an exposed surface of a PCB.
As integrated circuit fabrication technology improves, manufacturers are able to integrate more and more functions onto single silicon substrates. As the number of functions on a single chip increases, however, the number of signal lines which need to be connected to external devices also increases. The corresponding numbers of required I/O pads and device package terminals increase as well, as do the complexities and costs of the device packages. Constraints of high-volume PCB assembly operations place lower limits on the physical dimensions of and distances between device package terminals. As a result, the areas of peripheral-terminal device packages having hundreds of terminals are largely proportional to the number of terminals. These larger packages with fine-pitch leads are subject to mechanical damage during handling or testing. Mishandling can result in a loss of lead coplanarity, adversely affecting PCB assembly yields. In addition, the lengths of signal lines from chip I/O pads to device package terminals increase with the number of terminals, and the high-frequency electrical performance of larger peripheral-terminal device packages suffers as a result.
Controlled collapse chip connection (C4) is a well known method of attaching an integrated circuit chip directly to a PCB, and is commonly referred to as the "flip chip" method. In preparation for C4 attachment, the I/O pads of the chip are typically arranged in a two-dimensional array upon an underside of the chip, and a corresponding set of bonding pads are formed upon an upper surface of the PCB. A solder bump is formed upon each of the I/O pads of the chip. During C4 attachment of the chip to the PCB, the solder bumps are placed in physical contact with the bonding pads of the PCB. The solder bumps are then heated long enough for the solder to flow. When the solder cools, the I/O pads of the chip are electrically and mechanically coupled to the bonding pads of the PCB. After the chip is attached to the PCB, the region between the chip and the PCB is filled with an "underfill" material which encapsulates the C4 connections and provides other mechanical advantages.
Like flip chips, grid array semiconductor device packages have terminals arranged in a two-dimensional array across the underside surface of the device package. As a result, the physical dimensions of grid array device packages having hundreds of terminals are much smaller than their peripheral-terminal counterparts. Such smaller packages are highly desirable in portable device applications such as laptop and palmtop computers and hand-held communications devices such as cellular telephones. In addition, the lengths of signal lines from chip I/O pads to device package terminals are shorter, thus the high-frequency electrical performances of grid array device packages are typically better than those of corresponding peripheral-terminal device packages. Grid array device packages also allow the continued use of existing PCB assembly equipment developed for peripheral-terminal devices.
An increasingly popular type of grid array device package is the ball grid array (BGA) device package. A BGA device includes a chip mounted upon a larger substrate made of, for example, fiberglass-epoxy printed circuit board material or a ceramic material (e.g., aluminum oxide, alumina, Al.sub.2 O.sub.3, or aluminum nitride, AlN). The substrate includes two sets of bonding pads: a first set adjacent to the chip and a second set arranged in a two-dimensional array across the underside surface of the device package. Members of the second set of bonding pads function as device package terminals, and are coated with solder. The resulting solder balls on the underside of the BGA device package allow the device to be surface mounted to an ordinary PCB. The I/O pads of the chip are typically connected to corresponding members of the first set of bonding pads by signal lines (e.g., fine metal wires). The substrate includes one or more layers of signal lines (i.e., interconnects) which connect respective members of the first and second sets of bonding pads. During PCB assembly, the BGA device package is attached to the PCB by reflow of the solder balls just as a flip chip is attached to a PCB.
Semiconductor devices (e.g., integrated circuit chips) dissipate electrical power during operation, transforming electrical energy into heat energy. For high performance devices, such as microprocessors, specified performance is only achieved when the temperature of the device is below a specified maximum operating temperature. The heat energy produced by a semiconductor device during operation must thus be removed to the ambient environment at a rate which ensures operational and reliability requirements are met. The operating temperature of an integrated circuit chip enclosed within a semiconductor device package is thereby governed by: (i) the temperature of the ambient surrounding the device package, (ii) the amount of electrical power dissipated by the chip, and (ii) the sum of thermal resistances of elements and interfaces along a heat transfer path from the chip to the ambient. More complex heat transfer (i.e., cooling) mechanisms, such as heat sinks and forced air cooling, permit semiconductor devices to dissipate more electrical power than direct exposure to the ambient would otherwise allow.
The very short signal transition times of modern high frequency digital circuits also create potential problems. The short transition times require relatively large switching transient currents in order to charge and discharge capacitive loads quickly. The impedances of conductors carrying these large transient currents result in voltage transients. Such voltage transients result in ground "bounce", power supply "droop", and reduced noise margins of logic devices connected to these conductors. These voltage transients have significant high frequency components, and may be coupled into other circuit conductors located nearby. The high frequency components of these transient voltages also increase the levels of radiated electromagnetic interference (EMI).
It would be beneficial to have a packaged integrated circuit device including an integrated circuit chip enclosed within a semiconductor device package, wherein the device package includes grid array terminals, and wherein the chip is mounted upon a substrate of the device package using the C4 or flip chip attachment method. C4 attachment would beneficially reduce many of the problems associated with using fine metal wires to connect the I/O pads of the chip to corresponding bonding pads of the BGA package, including wire crossover problems and the added electrical inductances of the wires. It would be further desirable to surround the chip with electrically conductive elements, and connect these elements to a ground potential. Proper design of a ground about the chip will minimize radiation of electrical energy from, and coupling of electrical energy into, electrical circuits of the chip during operation.